The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Emissions control is an important factor in engine design and engine control. One particular combustion by-product, NOx, is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion. Rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. Reduction of NOx created in the combustion process and management of NOx in an exhaust aftertreatment system are priorities in vehicle design.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in exemplary devices known in the art within the broader category of aftertreatment devices. However, one having ordinary skill in the art will appreciate that aftertreatment devices are largely dependent upon operating conditions, such as device operating temperature driven by exhaust gas flow temperatures.
Modern engine control methods utilize diverse operating strategies to optimize combustion. Some operating strategies, optimizing combustion in terms of fuel efficiency, include lean, localized, or stratified combustion within the combustion chamber in order to reduce the fuel charge necessary to achieve the work output required of the cylinder. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust gas flow, can be greatly reduced from normal values. Such conditions can be challenging to exhaust aftertreatment strategies, since, as aforementioned, aftertreatment devices frequently require an elevated operating temperature, driven by the exhaust gas flow temperature, to operate adequately to treat NOx emissions.
Aftertreatment devices are known, for instance, utilizing catalysts to treat the exhaust gas flow and catalysts capable of storing some amount of NOx, and engine control technologies have been developed to combine these NOx traps or NOx adsorbers with fuel efficient engine control strategies to improve fuel efficiency and still achieve acceptable levels of NOx emissions. One exemplary strategy includes using a NOx trap to store NOx emissions during fuel lean operations and then purging the stored NOx during fuel rich, higher temperature engine operating conditions with conventional three-way catalysis to nitrogen and water. However, catalysts and NOx traps are dependent upon properties of the exhaust gas to operate efficiently. These methods can be temperature and engine range limiting. A selective catalytic reduction device (SCR) is known to additionally treat the exhaust gas flow utilizing a reductant, extending the aftertreatment capabilities of the aftertreatment system.
One known configuration of SCR utilizes ammonia derived from urea injection or recovered from normal operation of a three-way catalyst device as a reductant to treat NOx. Another known configuration utilizes a hydrocarbon selective catalytic reduction device (HC-SCR), wherein unburnt hydrocarbons, either injected in the exhaust gas flow or carried through from the combustion chamber, are utilized as a reductant to treat NOx. In either method, accurate dosing of the reductant is important to proper function of the device. Additionally, SCRs are dependent upon proper function of the catalyst within the device. Conditions can occur which reduce the efficiency of the catalyst. One particular example, in diesel applications, includes diesel fuel or sulfur poisoning of the catalysts. Such conditions can frequently be remedied, for example, through a regeneration cycle, when detected. One exemplary method to track and diagnose poisoned or otherwise malfunctioning catalysts is to monitor conversion of NOx within the device or the conversion efficiency of the device. Such diagnoses require monitoring of parameters indicative of NOx treatment.
A NOx sensor or an oxygen sensor add cost and weight to a vehicle, and such sensors frequently require a particular operating temperature range, achieved after some warm-up time, to be functional. There exist methods to estimate engine-out NOx via detailed combustion modeling using heat release model, multi-zone combustion model and Zodovich chemical kinetic equations. This detailed modeling, although it is good for analysis, may not be appropriate for in-vehicle engine control module (ECM) applications because of complicated programming and calibration requirements. Additionally, such models are sensitive to sensor tolerance and aging, pose a large computational burden upon the ECM, and require processing time not providing results in real-time.